Although some edible oils are used per se, by far the largest portion are hydrogenated, or hardened, prior to their end use. The reason for such hydrogenation is to increase the stability of the final product. For example, processed soybean oil is susceptible to oxidation resulting in deterioration of its organoleptic properties upon storage even at ambient temperature. Where the oil is to be used at higher temperatures, for example, as a frying oil, the adverse organoleptic consequences of oxidation become even more pronounced.
The commonly accepted origin of oxidative deterioration is the presence of highly unsaturated components, such as the triene moiety, linolenate, in soybean oil. Partial hydrogenation to remove most of this component leads to a marked increase in the oxidative stability of the resulting product, thereby facilitating storage and permitting unobjectionable use at higher temperatures. Ideally, one desires this hydrogenation to be highly specific, reducing only triene to the diene, linoleate, without effecting cis to trans isomerization. In practice, this goal is unachievable.
The fats and oils which are the subject of this invention, hereinafter collectively referred to as fatty materials, are triglycerides of fatty acids, some of which are saturated and some of which are unsaturated. In vegetable oils, the major saturated fatty acids are lauric (12:0), myristic (14:0), palmitic (16:0), stearic (18:0), arachidic (20:0), and behenic (22:0) acids. The notation, "18:0," for example, means an unbranched fatty acid containing 18 carbon atoms and O double bonds. The major unsaturated fatty acids of vegetable oils may be classified as monounsaturated, chief of which are oleic (18:1) and erucic (22:1) acids, and polyunsaturated, chief of which are the diene, linoleic acid (18:2) and the triene, linolenic acid (18:3). Unhardened vegetable fats and oils contain virtually exclusively cis-unsaturated acids.
In the context of partial hydrogenation, the ultimate goal is the reduction of triene to diene without attendant trans acid formation or saturate formation. In practice, it is observed that partial reduction results in lowering both triene and diene and increasing the monoene, saturate, and trans levels. Because it is desired that the product of partial hydrogenation itself be a liquid oil relatively free of sediment or even cloudiness upon storage at, for example, 10.degree. C., the formation of saturated and trans acids in such hydrogenation is a vexing problem. Removal of these solids, whose relative amount is measured by the Solid Fat Index (SFI), is a relatively costly and inefficient process attended by large losses associated with the separation of gelatinuous solids from a viscous liquid. It is known in the art that such solids are composed largely of triglycerides containing at least one saturated fatty acid moiety and/or trans monounsaturated fatty acid moiety with the predominant culprits having at least 18 carbon atoms. It is further known in the art that fatty acid analysis alone may be an insensitive analytical tool, that is to say, two products of hydrogenation of, for example, soybean oil may show different SFI profiles while having virtually identical fatty acid analysis. This arises because of the distribution of the saturated moieties in the triglyceride is important. The solubility in the soybean oil of disaturated triglycerides is much less than twice the amount of monounsaturated triglycerides, and the solubility of monounsaturated triglycerides may depend upon whether the other fatty acid moieties of the triglyceride are monounsaturated, diunsaturated, etc., and may also depend upon whether the saturated portion is at the one- or two-position of the triglyceride. Hence, hydrogenation of edible fats and oils is largely an empirical process, whose analytical tools include Solid Fat Index (SFI) supported by fatty acid analysis. The difficulty of achieving desirable results, in the context of selectivity in Solid Fat Index, has largely limited such hydrogenation to a batch type process. Although the transition from a batch to a continuous process, especially of the fixed bed type, is conceptually facile, it will be recognized by the skilled worker that impediments have been substantial.
Thus, U.S. Pat. No. 2,971,016 describes the vapor-phase hydrogenation of unsaturated fatty acids and esters in a fluidized bed, which enabled the disadvantages of liquid phase hydrogenation and the use of solid bed catalysts to be avoided. It will be recognized that vapor-phase hydrogenation is unfeasible for oils and fats. A continuous process based on a mixture of oil and suspended catalyst flowing along a tortuous path on the top surface of a series of perforated plates, with hydrogen admitted through the bottom face counter-current to the oil flow and minimum mixing along the various plates, is the subject of U.S. Pat. No. 3,634,471. The process described in U.S. Pat. No. 3,792,067, which has had limited commercial application, is based on a turbulent two-phase gas-liquid flow with minimal back-mixing, the liquid phase consisting of oil containing catalyst suspended therein. Both U.S. Pat. Nos. 3,823,172 and 3,988,329 describe continuous hydrogenation processes where the flowing mass of oil containing suspended catalyst is subject to high shear forces. U.S. Pat. No. 3,444,221 describes a continuous process which requires a high ratio of liquid (catalyst suspended in oil) to gas phase using a plurality of reaction chambers.
The processes represented in the latter four references all suffer from the common disadvantage of necessitating the additional unit process of removal of suspended catalyst from partially hydrogenated oil, as by filtration. It is well known in the art that this unit process entails substantial product loss and requires use of relatively large amounts of filter aid, which adds to processing cost and presents subsequent disposal problems. Because use of a fixed bed continuous operation obviates the necessity of catalyst removal, such a mode of operation is greatly preferred. Both U.S. Pat. Nos. 3,123,626 and 3,123,627 describe fixed bed processes using sulfur--or nitrogen-poisioned nickel respectively, on a macroporous silica support. At least in part, success of the method is attributable to the large pore structure of the support with catalyst contained within the pores. A diametrically opposed approach to fixed bed hydrogenation is described in U.S. Pat. No. 4,163,750, where metals, including nickel, are deposited almost entirely on the outer surfaces of the particles of the support. The support itself may be porous, and in fact advantages are ascribed to porous supports, such as porous carbon, as compared with non-porous supports, as stainless steel. The method of achieving surface deposition of the metal, which appears to be critical to the success of this process, does not seem to be disclosed.
The observation upon which the subject invention of this application is founded is that alpha-alumina of low surface area and low porosity functions as an effective support for catalytically active zerovalent nickel in a fixed bed hydrogenation of edible fats and oils, affording partially hydrogenated product with the desired selectivity. This observation seems unknown in the prior art, and stands in sharp contrast to the prior art requirements of a porous support. A further observation enhancing this invention is that the selectivity of hydrogenation of fatty materials is substantially improved when the fatty material is passed upflow over the fixed bed.
It is an object of this invention to provide a method of selective hydrogenation of fatty materials by a continuous process. An embodiment comprises hydrogenating a vegetable oil flowing upwardly over a fixed bed of a hydrogenation catalyst consisting essentially of catalytically active zerovalent nickel impregnated on alpha-alumina. In a more specific embodiment, the alumina has a surface area less than about 5 square meters per gram. In a still more specific embodiment, the nickel is present at a level from about 1 to about 25% based on alpha-alumina.